Why do some values on nodes lack an input socket?
The reason is that this is the way the node is written by the developer,
Sometimes it makes sense for values to be adjusted per-pixel and sockets are used, other times its enough to have a setting on the node.

The are undoubtedly cases where you might want to adjust a setting per-pixel but there isn't a socket input. I guess you could call this a not-yet-implemented feature. There may be a good reason the developer chose not to do this, or it may be an oversight.